Frequently Asked Questions about Gaia

The Mission

. Where does the name Gaia come from?

Gaia was originally the acronym for Global Astrometric Interferometer for Astrophysics, the initial name of the mission. This reflected the optical technique of interferometry that was first planned for use on the spacecraft. Although the acronym (initially written GAIA) is no longer applicable, the name remains (without the initial caps) to provide project continuity.

. What is the goal of Gaia?

The Gaia mission will survey about one billion stars in the Milky Way Galaxy to create the largest and most accurate three-dimensional map of the Galaxy ever obtained. In so doing, it will also detect new asteroids and extragalactic sources such as quasars, find new exoplanets and even provide a new test of Einstein’s theory of general relativity.

. Why is the mission important?

From the information obtained, we hope to understand much more about the structure, contents and evolution of our Galaxy, how it came into being and why it is the way it is.

. What are the primary mission objectives?

Gaia will attempt to measure the position and velocity of approximately one billion stars in the Milky Way – about 1% of the stars in the Galaxy – chart the three-dimensional distribution of these stars and determine their brightness, temperature, composition and motion through space.

. Are additional discoveries expected?

Scientists also expect to discover thousands of planets beyond the Solar System, tens of thousands of failed stars (brown dwarfs) and 20,000 exploding stars (supernovae). And by watching the large-scale motion of stars within the Galaxy, they also hope to probe the distribution of dark matter. Gaia will also measure some 500,000 distant quasars, providing a connection to the reference frame currently defined in radio wavelengths.

. When was Gaia started?

Gaia was approved in 2000 as a European Space Agency Cornerstone Mission within ESA’s Horizon 2000 Plus science programme. It is a purely European mission.

. Which ESA Member States are participating in this mission and how?

As an ESA Science Programme, contributions are mandatory, so all member states are taking part. Companies from 15 member nations have been awarded contracts to build the spacecraft (see brochure for the list). Most member states also have a role in the science portion of the mission as part of the Data Processing and Analysis Consortium (DPAC) created to handle mission data.

. Initially Gaia was supposed to be an interferometry mission. Why was this changed?

The interferometer concept was considered at the very inception of the mission but was quickly abandoned in favor of the current optical telescope design, which makes it possible to collect more signals and therefore to measure fainter stars.

. How will Gaia’s performance compare with previous astrometry missions like Hipparcos?

Hipparcos (1989-1993) catalogued more than 100,000 stars to a high precision and more than a million to lesser precision. Gaia will chart 10,000 times as many stars as Hipparcos, measuring their position and motion with 100 times greater accurately.

. How long will the mission last, and could it be extended?

After 4 months’ commissioning, the nominal mission will last 5 years, ending in 2018. Consumables (mainly fuel) have been sized to extend the mission by 1 year.

. How long beyond the nominal mission could Gaia expect to function, assuming all key systems remain in working order?

Assuming nominal fuel consumption, the mission could be extended a full year. However, actual fuel consumption in orbit may permit operation beyond this point: as a general rule, ESA science missions tend to last longer than originally designed.

. Why was it necessary to create the Data Processing and Analysis Consortium? Who is taking part, who financed it, and how long is it expected to function?

A primary motivation behind the Data Processing and Analysis Consortium (DPAC) is the unprecedented amount of data Gaia will generate: surveying 1 billion stars, 70 times each over five years amounts to an average of 40 million observations a day! This will translate into 40 Gigabytes of information flow/day, or 1 Petabyte (1 million Gigabytes) over the full life of the mission, equivalent to some 200,000 DVDs. Such a huge amount of data requires a vast range of scientific expertise that only networking can provide. DPAC will bring together more than 400 specialists from throughout the ESA community. It will remain in place around 3 years after the end of the mission, up to release of the final product: the Gaia catalogue.

. Is access to data limited to DPAC members? If not, how is access regulated?

Access to raw data is limited to DPAC members, who will be entrusted with the task of converting the telemetry data into scientifically meaningful information. This information will be released in the form of catalogues that will be made available to the worldwide astronomical community.

. When can we expect initial mission results to be made public?

The Gaia Intermediate Data Release scenario anticipates the first release to take place 22 months after launch. However, Science Alerts are expected to start as early as 2014.

. Will Gaia work in tandem with other current astronomy missions like Kepler or JWST?

We can expect that all Kepler stars will at some point in time obtain their distance data from Gaia. Similarly, the pointing for all visible JWST targets will most likely be based on coordinates provided by Gaia. It can also be assumed that some JWST proposals will be based on Gaia discoveries. However, beyond this no tandem working mode is anticipated.

. How will Gaia be launched?

The Gaia spacecraft will be launched by an STB/Fregat MT Soyuz rocket – a Europeanized version of Russia’s Soyuz – from Europe’s spaceport in Kourou, French Guiana.

. Initially the mission was supposed to be launched in 2010. Why the delay?

The reference launch date was 1 December 2011, the date formally established after selection of the prime contractor. (Previous dates, which did not take into account manufacturing details, were only indicative.) The current launch date is 20 December 2013, 2 years behind the initial schedule. Two months of this delay can be attributed to the need to fit mission liftoff into the overall launch schedule at Kourou. A further one month delay was due to an unexpected problem during the launch campaign. The remaining 21 months were essentially due to payload production complications, notably with respect to the focal plane and the 10 telescope mirrors.

. What is the total cost of the mission? How does the final cost compare to the initial cost forecast?

The total cost of the mission, from the beginning of preliminary studies to the end of operations, is 740 M€. This does not include expenditures related to the DPAC consortium, which are covered by the member states participating in the consortium, not ESA. These costs are currently estimated at around 200 M€. The price tag for the spacecraft itself is 450 M€. This is lower than the forecast at the very beginning, which was based on a larger spacecraft that would have been launched with an Ariane 5 rocket. Compared to estimates determined at the start of the industrial phase, mission cost will exceed the original forecast by 16%.

The Spacecraft

. What kind of instrument package does Gaia have, and how does it compare with those used in previous astronomy missions?

The instrument package comprises two identical optical telescopes/imaging systems, a radial velocity spectrometer and blue/red photometers. The telescopes feature a 106 CCD focal plane array with nearly 1 billion pixels – 1,000 times larger in size than a typical smart phone – making them the largest digital cameras ever used in space.

. What is the mission launch weight?

The Gaia spacecraft weighs 2,030 kg, including 710 kg of payload, a 920 kg service module (including the sunshield) and 400 kg of propellant.

. How big is the spacecraft?

The spacecraft measures more than 10 metres across with its sunshield/solar array assembly fully deployed.

. Who are the mission contractors?

Gaia was designed and built by Astrium, with a core team composed of Astrium France, Germany and UK. The industrial team included 50 companies from 15 European states, along with firms from the US. Some 80 contracts were placed with European companies and three with those in the US. The spacecraft will be launched by Arianespace. Between 2,500 and 3,000 people in all are involved in the mission.

. What were the most challenging engineering issues, and did any of them affect mission cost? Did any other factors impact the cost of the mission?

Gaia uses a highly precise cold gas micropropulsion system to maintain attitude control and keep the telescopes spinning at a constant rate. Difficulties involved in designing this and other key elements of the payload accounted for about two thirds of the cost overrun. The remainder was mainly due to an increase in the cost of the Europeanized Soyuz launch vehicle in the years since mission kickoff.

. In developing Gaia, has European industry gained technology expertise that can be reused in the commercial space arena? If so, in what areas?

. The telescope CCDs were delivered in 2008. Given the enormous technical progress in this area since then, are they still suitable for a mission in 2013?

Yes. The charge coupled device technology used for optical observation is still valid, and a Gaia designed now would use the same CCDs. Moreover, thanks to Gaia, we now have a much better understanding of CCD behavior in a radiation environment.

. What has been done to protect the CCDs from radiation damage?

From the very outset of the mission, the issue of CCD radiation sensitivity was known to be one of the key risk areas. ESA invested 3 M€ in development and testing to identify specific radiation risks and provide a way to obtain the performance required to mitigate against them. The solutions adopted include use of appropriate shielding, operation at very low temperatures and the introduction of special CCD operational modes. In addition, the effect of radiation on the CCDs was “calibrated” as a function of different operating parameters. These calibration figures will help ensure optimum performance in orbit.

. How will the mission function?

Gaia’s two telescopes will monitor each of its target stars about 70 times over a five-year period, spinning slowly to sweep the entire celestial sphere. As the telescopes repeatedly measure the position of each celestial object, they will detect the combination of the apparent motion caused by the parallax effect and the true motion of the object. By combining the measurements for all objects viewed, it will be possible to obtain the optimal parallax and proper motion for each object targeted.

. How accurate will Gaia’s measurements be?

Gaia will be able to measure celestial objects down to magnitude 20 and detect objects 400,000 times fainter than an unaided eye can see. It will measure the positions of objects of magnitude 15 or more with a precision of 24 microarcseconds or better, comparable to gauging the diameter of a human hair at a distance of 1,000 km. Accuracy will range from 20% for stars near the centre of the Galaxy, some 30,000 light-years away, to a remarkable 0.001% for the stars nearest to our Solar System.

. In what orbit will Gaia operate?

The spacecraft will circle the Sun in a Lissajous-type orbit at the L2 Lagrangian point, 1.5 million km from Earth. This location permits a clearer view of the cosmos than an orbit around Earth, where it would pass in and out of the Earth’s shadow, distorting its view.

. Where will mission operations be managed?

The mission will be controlled from the European Space Operations Centre (ESOC) in Darmstadt, Germany using ground stations in Cebreros, Spain and New Norcia, Australia. Science operations will be conducted from the European Space Astronomy Centre (ESAC) in Villafranca, Spain.

The launch and beyond

. Why did you choose the Russian Soyuz launcher and not the preferred European launch system, the Ariane 5?

The cost of the Ariane 5 is more than twice that of Soyuz, which makes the Russian launch vehicle significantly more attractive. Gaia was subsequently sized so it could be launched with Soyuz.

. If the launch cannot take place as planned, how large is the launch window?

The launch window opens on 17 December and closes on 5 January 2014, which means there is a 15 day margin with respect to the planned 20 December launch date. The next available launch window opens on 16 January and closes on 3 February 2014.

. What are the chief launch risk elements, and what are the planned fallback scenarios in the event of a degraded mission?

One of the most critical elements in the mission is the sunshield, which permits the telescopes to operate at a suitably cold temperature (-110°C) and, thanks to solar panels mounted on its undershield, serves to power the spacecraft and its instruments. Due to its large size, the sunshield had to be designed in multiple panels that unfold once the spacecraft is in transfer orbit. Mission success depends on its successful deployment.

Other critical elements include the payload bipods, which serve to take up payload stress during launch and must be released once the spacecraft reaches transfer orbit; the Day 2 manoeuvre two days after launch, when the spacecraft is placed on course to L2; and insertion into L2 orbit. If the bipods fail to release, the mission can continue but with some loss of capability. If the Day 2 and L2 manoeuvres fail to take place exactly as planned, there will also be a loss of mission capability, depending on how much fuel is expended in completing them correctly.

. Is the launch insured?

In line with general ESA policy with respect to science missions, the launch is not insured.

. In the event of a launch failure, will you be able to rebuild the spacecraft, and if so, how long would this take?

Since the launch is not insured, construction of a new spacecraft would depend on the willingness of ESA members states to fund it (as was done in the case of Cluster). This would probably take around 5 years, key schedule drivers being the time needed to produce the 106 CCDs and the 10 mirrors required for the mission.

. What will happen if one or more of the instruments on-board fail? What kind of failure would threaten the success of the mission?

All spacecraft systems are redundant, which means that the mission would be lost after a double failure of the same unit. However, the critical focal plane is built in seven independent rows, meaning seven units would have to fail for the plane to be lost. In addition, each of the chief launch risk failures mentioned above could be mission threatening.